The invention relates to a method and an apparatus for controlling the proportion of fuel and air in a combustible fuel-air mixture fed to an internal combustion engine. More particularly, the invention relates to an apparatus in which an oxygen sensor (.lambda.-sensor) monitors the exhaust gas composition and generates a signal which is used in influencing the fuel-air ratio. For this purpose, the sensor signal is compared with a set-point or threshold value.
Known in the art are systems which determine the duration of fuel injection control pulses by disposing in the exhaust system a .lambda.-sensor which generates an electrical signal that alternates abruptly between a higher and lower voltage depending on whether the mixture fed to the engine is rich or lean. This output signal is used as the actual value in a control loop and is used by the fuel injection system to determine the duration of the control pulses used to actuate the injection valves. The basic duration of the fuel injection control pulses is determined on the basis of two major variables, i.e., the engine rpm and the air flow rate aspirated by the engine. The fuel injection control pulses are generated in synchronism with crankshaft rotations. In this previously proposed system, an attempt is made to maintain the .lambda. control in the critical temperature domain, where the sensor has a very high internal resistance and is capable only to generate signals which are substantially shifted in voltage, by permitting the threshold or set-point voltage with which the sensor output is compared to follow the changing sensor potential. In this process, however, considerable non-linearities are produced. It is also particularly disadvantageous that aging an a dispersion of the characteristics of the sensor make the adjustment and the control process very difficult in this critical temperature domain.